Recharge iPhone 12 overnight

It’s a best practice and the absolute best way to slow the decline of battery capacity long term; enable Optimized Battery Charging (Settings/Battery/Battery Health) and charge the device overnight, every night. The battery will fast charge to 80%, then pause. During the nighttime pause the phone will use mains power instead of battery power, allowing the battery to “rest”, and thus reducing the need to charge the battery quite as often. The phone will resume charging to reach 100% when you are ready to use your phone; it will “learn” your usage pattern. If you enable iCloud Backup (Settings/[your name]/iCloud - iCloud Backup) the phone will back up overnight also, assuring that you can never lose more than the current day’s updates.

Yes, in fact most users charge overnight so the phone will be ready for use the next day. You cannot overcharge the lithium ion battery. Check this link...

https://www.apple.com/batteries/maximizing-performance/